Hydrogenation of oxidative dehydrogenation by-product

ABSTRACT

A by-product stream containing oxygenated hydrocarbons including furan resulting from hydrocarbon oxidative dehydrogenation processes is upgraded in value and rendered readily separable from residual close-boiling hydrocarbons remaining in the byproduct stream by subjecting the by-product stream to hydrogenation to convert furan to tetrahydrofuran and then separating the resulting tetrahydrofuran by fractionation from the hydrogenation effluent.

United States Patent Cottle [451 Sept. 19, 1972 [54] HYDROGENATION OFOXIDATIVE 3,518,284 6/1970 Foster ..260/680 X DEHYDROGENATION BY-PRODUCT3,238,225 3/1966 Brill et a1 ..260/346.1 72 Inventor: J E. om l PhillipsPetrole 2,846,449 8/1958 Banford t 8.1 ..260/346.1

um Company, Bartlesville, Okla. 74003 Primary Examiner-Paul M. Coughlan,Jr. 221 Filed: March 13, 1970 Mama-hung [21] Appl. No.: 19,265 [57]ABSTRACT A by-product stream containing oxygenated hydrocar- [52] US.Cl. ..260l680 E, 203/32, 260/346.l R bons including furan resulting fromhydrocarbon ox- [51] Int. Cl. ..C07c 5/18, C07d 5/02 idativedehydrogenation processes is upgraded in Field of Sara "260/630 346-1 R;2033/32 value and rendered readily separable from residual close-boilinghydrocarbons remaining in the by- [56] Reerences Cited product stream bysubjecting the by-product stream to UNITED STATES PATENTS hydrogenationto convert furan to tetrahydrofuran and then separating the resultingtetrahydrofuran by frac- 2,991,320 7/1961 Heame et a1. ..260/680tionation from the hydrogenation ffl t 3,320,329 5/1967 Nolan ..260/6802,725,344 11/1955 Fenske et a] ..260/346.1 5 Claims, 1 Drawing FigureLIGHTS 34 AIR 2 13 I9 '2 11 g 37 36 57 56 66 67 g E 5 HC FEED a: '8 Q 5464 1 1 E THF PRODUCTS 14 I5 c WATER g 32 g o 1- 53 63 IO E Z; L Zt n I 2E 2 g g g z m o O 3 5 F E s 2 2 E E 62 E 73 7O HEAVIES HYDROGENATION OFOXIDATIVE DEI'IYDROGENATION BY-PRODUCT BACKGROUND OF THE INVENTION Thisinvention relates to the purification and separation of a by-productstream obtained as an effluent from an oxidative dehydrogenationprocess. In accordance with another aspect, this invention relates tothe hydrogenation of a by-product stream obtained from an oxidativedehydrogenation process obtaining oxygenated compounds including furan,which is difficultly separable from associated close-boilinghydrocarbons including acetylenes and dienes, to convert the furan totetrahydrofuran which is readily separable by fractionation. Inaccordance with a further .aspect, this invention relates to'theoxidative dehydrogenation of hydrocarbon followed by the separation ofunconverted dehydrogenatable hydrocarbon and dehydrogenated product,e.g., butadiene, leaving a narrow-boiling, heavy fraction containingfuran, butyne-2, 2-pentene, piperylenes, and isoprene, hydrogenatingthis narrow-boiling, heavy fraction to convert the furan totetrahydrofuran and fractionating from the hydrogenation effluent a highpurity tetrahydrofuran product.

It is conventional in the petroleum industry to catalyticallydehydrogenate n-butane over a catalyst such as iron oxide deposited onan alumina base or carrier to produce an effluent comprising butenes andbutadiene. The resulting effluent with or without intermediateseparation steps is then subjected to further dehydrogenation in contactwith a butene dehydrogenation catalyst to convert the butenes tobutadiene, the latter being separated as a product.

A conventional catalyst for butene dehydrogenation comprises iron oxide,chromium oxide, and an alkali metal carbonate, such as potassiumcarbonate. Operation with this catalyst in the presence of relativelylarge concentrations of steam promotes the water-gas reaction andmaintains the catalyst substantially free of carbonaceous deposits aslong as there is an effective concentration of the carbonate present inthe catalyst.

In a more recent butene dehydrogenation process known as oxidativedehydrogenation, an oxygen-containing gas is fed to the catalyticreaction zone containing a catalyst such as stannic phosphate along withthe butene feed and steam, and a substantial portion of the hydrogenproduced by dehydrogenation is combusted to water vapor. This not onlyremoves the inhibiting ef fect of the hydrogen on furtherdehydrogenation, but also supplies heat to this endothermic reactionresulting in high conversions and per-pass yield of butadiene atrelatively good selectivity. By this method, additional steam isproduced which is recovered from the process effluent as condensate.Also, moderate concentrations of oxygenated hydrocarbons are generatedwhich similarly appear in the condensed steam and/or in the hydrocarboneffluent.

, A highly reactive dehydrogenation catalyst comprising tin phosphatehas recently been disclosed and claimed by Nolan in US. Pat. No.3,320,329. As is set forth in that patent, compounds to bedehydrogenated, preferably selected from the group consisting of a1-kenes, cycloalkenes, alkylpyridenes and alkyl aromatics, are mixed withoxygen or an oxygen-containing gas, preheated, and passed over acatalyst comprising 0 number of oxygenated products of the reaction.This residual oxygen and the oxygenated compounds are corrosive and aresubject to polymerization and are therefore detrimental to the furtherprocessing of the hydrocarbon product. It is therefore necessary thatthe oxygen and oxygen-containing compounds be removed from the effluentstreams.

It has been found that a'small percentage of the olefin feed isconverted to oxygenated hydrocarbons such as furan, alcohols, acids,aldehydes, ketones, etc., the nature and quantity of these compoundsdepending upon the conditions under which the dehydrogenation iseffected. Under normal plant operating conditions, these oxygenatedby-products will be ultimately fed into the atmosphere and/or dischargedwith waste water and/or end up in a heavy hydrocarbon-containingfraction, depending upon the separation and recovery processes employedand their operating conditions.

The present invention is directed to the purification from anarrow-boiling fraction comprising furan and residual close-boilinghydrocarbons remaining following separation of the oxidativedehydrogenation effluent whereby the furan is upgraded to a morevaluable product, which product is readily separable from the othermaterials present in the narrow-boiling fraction.

Accordingly, an object of this invention is to provide an improvedprocess for the oxidative dehydrogenation of hydrocarbons.

Another object of this invention is to provide a process for the removalof oxygen-containing compounds from oxidative dehydrogenation effluents.

A further object of this invention is to provide a process for thepurification of hydrocarbon streams containing oxygen-containingcompounds including furan.

A further object of this invention is to provide a process for renderingnarrow-boiling fractions readily separable by fractionation.

Other objects and aspects, as well as the several advantages of theinvention, will be apparent to those skilled in the art upon reading thespecification, the drawing and the appended claims.

SUMMARY OF THE INVENTION In accordance with the invention, anarrow-boiling fraction containing furan and close-boiling hydrocarbonsincluding acetylenes and dienes is rendered readily separable byfractionation by subjecting the narrowboiling fraction to hydrogenationto convert the furan to tetrahydrofuran.

- In accordance with one embodiment of the invention, a heavy bottomsphase which is a narrow-boiling fraction containing furan, butyne-2,Z-pentene, piperylenes, and isoprene is subjected to hydrogenation toconvert the furan to tetrahydrofuran and the effluent from thehydrogenation is subjected to fractionation to recover as product a highpurity tetrahydrofuran stream.

In accordance with a further specific embodiment of the invention, theeffluent from a butenes oxidative dehydrogenation is first subjected tofractionation to recover unconverted butenes and butadiene product,

' recovered overhead from the second stage, and the remaining heavymaterials as bottoms from the second fractionation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ln carrying out the invention,hydrocarbons are dehydrogenated in the presence of oxygen with theresult that a minor amount of hydrocarbon feed is converted tooxygenated derivatives thereof to produce an effluent comprisingreactant hydrocarbons, dehydrogenated hydrocarbons, oxygenatedhydrocarbons and water. The reactant hydrocarbons and dehydrogenatedhydrocarbons are separated from the remainder of the mixture leaving awater phase containing some oxygenated hydrocarbons and a hydrocarbonphase containing furan, the latter being a narrowboiling fraction whichalso contains compounds having overlapping boiling points such asbutyne-2, isoprene, 2-pentene, piperylenes, and n-pentane.

The following tabulation of boiling points for the components of thenarrow-boiling fraction illustrates the difficulty in recovering any ofthe components in a state of high purity by ordinary fractionation:

Component Boiling Point, F

lsopentane 82.1 Butyne-2 80.6 Furan 89.6 lsoprene 93.3 n-Pentane 96.9Pentene-Z 98 3-Methylbutadienel ,2 I04 cis-Piperylene l l l .6transPiperylene 108.! Tetrahydrofuran l5l Furan is actually moredifficult to fractionate from admixture with the components listed abovethan the boiling points illustrate because the components do not form anideal solution, i.e. the volatilities of the components may be increasedor decreased in the presence of the other components thus leading to 2or more components which form constant-boiling mixtures. It is apparent,however, that tetrahydrofuran is more easily fractionated from thelisted hydrocarbons.

The compounds most suitable for use in the process of oxidativedehydrogenation are compounds selected from the group consisting ofalkenes, cycloalkenes, alkylpyridines and alkyl aromatics. Thesecompounds are dehydrogenated at relatively high conversion andselectivity rates with and without the use of steam. Hydrocarbons thatcan be oxidatively dehydrogenated preferably according to the processinclude those having two to 12 carbon atoms as represented by propane,butane, butene, pentane, pentene, hexane, hexene, etc. Of specialinterest is the dehydrogenation of butenes to butadiene and thedehydrogenation of isoamylenes to isoprene. Catalysts that can beemployed include stannic phosphate, iron stannate or stannite,tin-aluminum phosphate, etc. The amount of oxygen (air can be used)employed during this dehydrogenation ordinarily will be in the range of0.1 to 3 volumes per volume of gaseous hydrocarbon feed. The amount ofsteam added to the feedstock passed to the dehydrogenation reactor willbe in the range of 0.1 to volumes per volume of gaseous hydrocarbon. Thetemperature for dehydrogenation will ordinarily be in the range of 800to l,200 F. The presently preferred hydrocarbons for dehydrogenation arethe C4 and C5 hydrocarbons, essentially the butenes and isoamylenes. Theoxidative dehydrogenation process is well known in the art and isdescribed in detail in U.S. Pat. No. 3,320,329.

A better understanding of the invention will be obtained upon referenceto the accompanying drawing which illustrates a preferred embodiment ofthe invention in combination with the processing following butenesoxidative dehydrogenation.

Referring to the drawing, air by line 11 and hydrocarbon feed by line 12are introduced into oxidative dehydrogenation reactor 13. The conditionsfor carrying out oxidative dehydrogenation are set forth above and aredescribed in detail in US. Pat. No. 3,320,329.

The oxidative dehydrogenation effluent comprising unreactedhydrocarbons, dehydrogenated hydrocarbons, oxygenated hydrocarbons,steam, oxygen and nitrogen is removed from reactor 13 via pipe 14 andpartially cooled, for example from 1, 1 25 to about 400 F., by passagethrough heat exchanger 15 wherein it boils water to form steam. Thegaseous effluent from exchanger 15 is additionally cooled to about 240F. by passage through water quench zone 16 to which cold water is passedvia pipe 10. An aqueous phase containing a predominance of the heavieroxygenated hydrocarbons contained in the reactor effluent is removedfrom zone 16 by way of pipe 17. The remaining bulk of the reactoreffluent, still in the gaseous state, is passed to separation zone 19which consists of a conventional mineral seal oil absorption andstripping steps as illustrated in more detail in US. Pat. No. 2,963,522.The oil absorber serves to separate the light gases such as oxygen,nitrogen (from the air passed to reactor 13), methane, hydrogen, etc.from the heavier hydrocarbons and oxygenated compounds by selectiveabsorption of the latter in the oil. The light gases are thus rejectedvia pipe 20 while the heavier hydrocarbons and oxygenated compounds arepassed via pipe 25 to fractionator 26.

A stream consisting essentially of butadiene and unreacted butenes isremoved overhead from fractionator 26 via pipe 27 and sent toadditionalsteps for recovering the butadiene product. Bottoms from fractionator 26consist of furan, isoprene, piperylenes, pentene-2, and heavierhydrocarbons and oxygenated compounds and is passed via pipe 32 tofractionator 33 for removal of the heavier components. Fractionator 33is operated with sufficient reflux and contacting trays that theoverhead stream 34 is composed of a narrow-boiling fraction containingprincipally furan, isoprene, butyne-2, piperylenes, and pentene-2.

A suitable upper temperature in fractionation zone 33 is about 100 F.with a bottoms temperature of about 220 F. and a column pressure ofabout 25 psig. A heavy bottoms fraction comprising some furan, butmostly heavier material, is removed by way of line 35 and passed tostorage or other suitable use. A portion of the bottoms stream 35 ispassed through reboiler 22 and reintroduced into a lower portion ofcolumn 33 as a source of heat.

The overhead fraction in line 34 is passed through condenser 37 andthence to accumulator 38. A portion of the condensate removed fromaccumulator 38 is passed as reflux by way of line 39 to an upper portionof column 33.

The remainder of the condensate comprising furan and close-boilinghydrocarbons removed from accumulator 38 is passed by way of line 36through pre-heater 41 and introduced into line 42 for introduction intohydrogenation reaction zone 46. Fresh hydrogen which can be either purehydrogen or a hydrogen-containing stream is introduced into line 42 byway of line 43 at about the hydrogenation reaction zone temperature.Also introduced into 42 is a vaporous recycle stream containing hydrogenobtained from the effluent of the hydrogenation reaction zone 46.Additionally, a portion of the liquid recovered from the hydrogenationeffluent is recycled to feed line 40 by way of line 44 for temperaturecontrol.

Hydrogenation reaction zone 46 in the specific embodiment is operated atan inlet temperature of about 200 F. and 225 psia and an outlettemperature of about 280 F. The furan introduced into hydrogenationreactor 46 is converted substantially to tetrahydrofuran. The totalhydrogenation reactor 46 feed, which includes hydrocarbon obtained frompre-fractionator 33 as well as recycled product and fresh and recycledhydrogen, is introduced by way of line 47 into the reactOI.

As indicated above, the hydrogen can be added in the form of purehydrogen, a hydrogen gas diluted with an inert diluent such as nitrogen,or any other hydrogen-containing gas. Reaction of hydrogen with theoxygen-containing materials, and especially furan, occurs at atemperature of from about 100 to about 400 F preferably from about 200to about 300 F and a pressure of from about to about 1,000 psia,preferably about 100 to about 300 psia. The temperature, of course,depends on whether a catalyst is employed and the type of catalystemployed, if any.

The hydrogen addition can be conducted without catalysts, but metalcatalysts containing metal atoms of cobalt, chromium, copper, iron,molybdenum, nickel, palladium, platinum, rhenium, rhodium or zinc can beused with suitable supporting or diluting materials such as alumina,carbon, silica or other similar materials.

The hydrogenation effluent which comprises principally hydrogen, butane,isopentane and tetrahydrofuran is removed by way of line 48, cooled byheat exchanger 49 and passed by way of line 50 to product separator 51.Uncondensed materials including hydrogen removed from the hydrogenationeffluent are separated from product separator 51 and returned tohydrogenation reactor 46 as recycle by way of line 45. The productseparator can be operated at a temperature of about F. and 210 psia inthe embodiment described. Product liquid which comprisestetrahydrofuran, isopentane and butane and a small amount of hydrogenand other materials is removed from separator 51 by way of line 52. Aportion of this liquid product removed by line 52 is recycled by way ofline 44 as a portion of the feed for hydrogenation reaction zone 46 andis introduced into line 40. The amount of recycle, which stream containsa substantial amount of tetrahydrofuran, is controlled so as to preventrunaway hydrogenation temperatures in zone 46. The ratio of recyclestream 44 to make-up stream 36 is typically in the range of 2 to 10 molsof stream 44 per mol of stream 36, more generally about 5 mols of stream44 per mol of stream 36.

The remainder of the liquid removed from separator 51 is passed by wayof line 53 as feed to fractionation zone 54. Fractionation zone 54 isoperated at an upper temperature of about 1 15 F. and a bottomstemperature of about 200 F. An overhead fraction comprising isopentaneand butane is removed overhead by way of line 55, passed throughcondenser 56 and thence to accumulator 57. A portion of the condensateremoved from accumulator 57 is returned as reflux to an upper portion ofcolumn 54 by way of line 58. The remainder of the hydrocarbons takenoverhead is removed for further use as desired by way of line 59.

A bottoms product comprising principally tetrahydrofuran is removed fromthe bottom of fractionation zone 54 by way of line 60. A portion of thebottoms product is passed through reboiler 61 and introduced into alower portion of column 54 by way of line 62 as a source of heat for thecolumn. The remainder of the bottoms product removed from column 54 ispassed by way of line 63 to fractionation column 64. Column 64 isoperated with an upper temperature of about F. and a bottom temperatureof about 205 F.

An overhead fraction comprising tetrahydrofuran is removed by way ofline 65, passed through condenser 66 and thence into accumulator 67. Aportion of the condensate removed from accumulator 67 is returned asreflux to an upper portion of column 64 by way of line 68 and theremainder, a high purity tetrahydrofuran product, is removed by way ofline 69.

A bottoms fraction comprising some tetrahydrofuran and heavier materialsis removed from the bottom of column 64 by way of line 70 and passed forfurther use as desired by way of line 71. A portion of the bottomsstream is passed to reboiler 72 and introduced into a lower portion ofcolumn 64 by way of line 73 as a source of heat.

The tetrahydrofuran product recovered overhead from column 64 is about99 percent pure and as illustrated by the above description it isreadily separable from the associated hydrocarbons present in the mix- 78 ture in which furan is obtained as a by-product from a jected tofractionation in column 33,'hydrogenation in butenes oxldativedehydrogenation process. The narreactor 46 using a hydrogenationcatalyst comprising row-boiling fraction obtained from the butenesoxida- 65 wt percent nickel on Kieselguhr support obtained tivedehydrogenation contains butyne-2 (boiling point from Harshaw ChemicalCompany as catalyst l404T. 80.6 F.) and isoprene (boiling point 93.3 F.)in addi- 5 The catalyst was activated by passage hydrogen tion to furan(boiling point 89.6 F.). In view of the through it at 300 to 400 F. forabout 6 hours. The

closeness of the boiling points, it can be readily seen hydrogenationeffluent was separated and a liquid that furan would be very difficultto recover in high puproduct subjected to two-stage fractionation inrity by ordinary fractionation because of the presence columns 54 and64. of the close boiling materials. However, in accordance Fractionator33 is 8 inches in diameter and contains with the invention wherein thefuran is converted to 30 feet of 26-inch Raschig rings as fractionationtetrahydrofuran (boiling point 151 F.), the packing. It is operated at25 psia, a top temperature of tetrahydrofuran is readily separable byconventional M and a bottom mp u 0 fractionation as illustrated by thedrawing and as Fractionator 54 is the same as fractionator 33 in sizedescribed above. and fractionation packing and is also operated at 25psia. It has top temperature of 116 F. and a bottom temperature of 198F. SPECIFIC EXAMPLE Fractionator 64 is l-foot in diameter and contains41 Utilizing a flow substantially as describe abOVe in feet of 56-inchRaschig rings as fractionation packing. It connect on with he accompnying draw g. a se operates at a pressure of psia, a top temperature ofboiling heavy fraction containing furan and various 197 F and a bottomtemperature of 205 F. hydrocarbons including acetylene (butyne-2) and Aportion of the product was recycled to the reactor diene (isoprene) wasobtained from the effluent from a 46 as was hydrogen recovered from theeffluent. The

butenes oxidative dehydrogenation. The bottoms flows based on mols perhour for the various com- 25 product, which was a narrow-boilingmixture, was subponents in the various streams are set forth In Table I.

TABLE I Mols per hour Stream N0 32 as as 43 4s 42 44 47 Fresh CombinedHydrofeed Column Column Column Hydrogen hydrogen carbon plus ReactorStream component 33 feed 33 KP 33 OHP Hydrogen recycle feed recyclerecycle feed Hz 21. 3979 1. 7979 23. 1958 1. 4779 1. 4779 24. 6737Methane 0 0082 0117 Butene- 7951 7951 n-Butane 12. 4844 12. 5995Butadiene 0580 0580 Pentene-L. 0699 0699 Isoprene 6407 6407'Iraris-piperylene 0583 0583 Cis-piperylene 0233 0233 Pentene-2 02320232 Isopentane 4. 8124 4. 8370 n-Pentane 1630 16303-methylbutadiene-l,2 0349 0349 Butyne-2 0288 0288 Furan 9. 4643 9. 4658Heavies 1422 1422 n-Butyl alcohol 6021 6023 Stream N0 48 52 53 59 63 69Product Reactor separator Column Column Column Column Column Streamcomponent efiluent liquid 54 feed 54 OHP 64 feed 64 KP 64 OHP .0117 0092.0010 .0010 Butene-2 Butadiene Pentene-l Isoprene Trans-p1 peryleneCis-piperylene. Pentene-Z. Isopentana. n-Pentane Heavies 0155 n-Butylalcohol 7291 7289 1268 1237 .1207

Tetrahydrofuran 52. 1917 52. 1354 9. 0666 1808 8. 8858 4905 8. 2953Total mols/hr 76, 2660 74. 2679 12. 9187 3. 9018 9. 0169 6186 8. 3983Total 1bs./hr 5, 059. 05 6, 042. 74 877. 17 226. 71 650. 46 44. 605. 56

N0'1E.KP Kettle product. 0 H P O verhead product.

lclaim:

l. A process for the oxidative dehydrogenation of dehydrogenatablehydrocarbons which comprises:

a. contacting C and C hydrocarbons with an oxygen-containing gas and adehydrogenation catalyst to produce a reaction effluent comprisingdehydrogenated hydrocarbon, unconverted hydrocarbon and oxygenatedcompounds including furan,

b. separating dehydrogenated hydrocarbon and unconverted hydrocarbonfrom said effluent, leaving a heavy bottoms close boiling fractioncontaining furan and other materials including residual amounts ofdehydrogenated hydrocarbon including dienes and acetylenes andunconverted hydrocarbons,

c. passing said heavy bottoms close boiling fraction obtained in step(b) to a fractionation zone and removing as bottoms a heavy fraction andan overhead fraction comprising furan and diene and acetylenehydrocarbons,

. passing said overhead fraction obtained in step (c) to a hydrogenationzone and therein contacting same with hydrogen and a hydrogenationcatalyst to convert furan to tetrahydrofuran and dienes and acetylenesto saturated hydrocarbons and produce a hydrogenation effluentcomprising tetrahydrofuran, saturated hydrocarbons and hydrogen,

e. separating hydrogen from said effluent obtained in step (d) andrecycling same to the feed to said hydrogenation zone, leaving a liquidphase comprised of saturated hydrocarbons and tetrahydrofuran, and

f. subjecting said liquid phase obtained in step (e) to fractionation toseparate same into saturated hydrocarbon fractions and a tetrahydrofuranfraction as product.

2. A process according to claim 1 wherein the liquid phase obtained instep (e) is subjected to two-stage fractionation wherein light saturatedhydrocarbons are recovered overhead in the first stage, tetrahydrofuranis recovered overhead in said second stage, and heavy saturatedhydrocarbon materials are recovered as hottoms from the second stage.

3. A process according to claim 1 wherein the hydrocarbon to bedehydrogenated is a C hydrocarbon-containing feed rich in butenes andwherein the heavy bottoms fraction obtained in step (b) contains, inaddition to furan, butyne-2 and isoprene, both of which have boilingpoints close to that of furan and which are difficultly separatabletherefrom by fractionation.

4. A process according to claim 1 wherein the effluent removed from thehydrogenation in step (d) is cooled prior to fractionation to condenseliquefiable liquids in the effluent and the cooled effluent is thensubjected to gaseous liquid separation wherein the gaseous fractioncomprising hydrogen is recycled to the hydrogenation zone and the liquidfraction is subjected to fractionation to separately recover light andheavy saturated hydrocarbons and tetrahydrofuran as product.

5. A process according to claim 4 wherein the fractionation is effectedin two stages wherein light saturated hydrocarbons are recoveredoverhead from the first fractionation stage, tetrahydrofuran isrecovered as the overhead from the second fractionation stage, and theheavy saturated hydrocarbons are recovered as bottoms from the secondfractionation stage.

separate

2. A process according to claim 1 wherein the liquid phase obtained instep (e) is subjected to two-stage fractionation wherein light saturatedhydrocarbons are recovered overhead in the first stage, tetrahydrofuranis recovered overhead in said second stage, and heavy saturatedhydrocarbon materials are recovered as bottoms from the second stage. 3.A process according to claim 1 wherein the hydrocarbon to bedehydrogenated is a C4 hydrocarbon-containing feed rich in butenes andwherein the heavy bottoms fraction obtained in step (b) contains, inaddition to furan, butyne-2 and isoprene, both of which have boilingpoints close to that of furan and which are difficultly separatabletherefrom by fractionation.
 4. A process according to claim 1 whereinthe effluent removed from the hydrogenation in step (d) is cooled priorto fractionation to condense liquefiable liquids in the effluent and thecooled effluent is then subjected to gaseous liquid separation whereinthe gaseous fraction comprising hydrogen is recycled to thehydrogenation zone and the liquid fraction is subjected to fractionationto separately recover light and heavy saturated hydrocarbons andtetrahydrofuran as product.
 5. A process according to claim 4 whereinthe fractionation is effected in two stages wherein light saturatedhydrocarbons are recovered overhead from the first fractionation stage,tetrahydrofuran is recovered as the overhead from the secondfractionation stage, and the heavy saturated hydrocarbons are recoveredas bottoms from the second fractionation stage.